there are File [https://en.wikipedia.org/wiki/Comparison_of_download_managers Download Manager type of software], please use one of those to download Thunderbird installer. these software can resume download, where it stopped download. after downloading installer file , please use [https://en.wikipedia.org/wiki/Comparison_of_software_calculators Hash/checksum/integrity/digest CALCULATING software], to find out downloaded file's hash integrity code, then compare calculated integrity code , with other (same category) integrity code which is shown in original/upstream software's website, (which is under their primary domain-name (not under any sub-domain), and you viewed/obtained website over HTTPS/encrypted connection). Thunderbird = TB. [https://archive.mozilla.org/pub/thunderbird/releases/ TB DOWNLOAD LINK]. there you will also see other files related to checksum/integrity.

you may also see bottom side of this ANSWER, [https://stackoverflow.com/a/64980413/3553808 here] . that part also shows various download links for Thunderbird & Firefox.